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1. The cholinergic differentiation of two phenotypically muscles of the chick, the slow multiply innervated anterior latissimus dorsi (a.l.d.) and the fast focally innervated posterior latissimus dorsi (p.l.d.), was investigated during embryonic life and after hatching using both autoradiographical and biochemical methods. 2. The contents in total protein and in acetylcholinesterase activity follow similar development patterns in both muscles, but, after the 15th day in ovo, the accumulation of choline acetyltransferase activity and of acetylcholine nicotinic receptor sites as determined by alpha-bungarotoxin binding occurs at a faster rate in a.l.d. than in p.l.d. 3. In muscle of the p.l.d., a rapid increase of the total number of acetylcholine receptor clusters takes place after the 11th day of embryonic life although some clusters could be observed on myofibres as soon as the 4th day in ovo. 4. The rate of degradation of cholinergic receptor sites in chick muscle is constant around 28 hr up to the 10th day after hatching; thus the different rates of accumulation of acetylcholine receptor in a.l.d. and p.l.d., respectively, after the 15th day of embryonic life must be due to different rates of receptor synthesis. 5. The role of muscle activity in the biochemical differentiation of the developing motor end-plate was investigated in chick embryos which had been paralysed by repeated injections into the yolk sac of a curare-like agent, Flaxedil (May & Baker). 6. The total content in acetylcholinesterase of both a.l.d. and p.l.d. muscles is not significantly modified by paralysis. However, the histochemical staining of end-plates for acetylcholinesterase as well as the heavy form of this enzyme (19 . 5 S) are consistently reduced after Flaxedil injection. 7. In muscles from Flaxedil-treated embryos, the total content in acetylcholine receptor sites as determined by alpha-bungarotoxin binding is higher than in those from control embryos, whereas the rate of degradation of these sites is not significantly altered. 8. The localization of the acetylcholine receptors under the motor nerve terminals is not prevented by blocking muscle activity at the postsynaptic level. Clusters of receptor are still present, and there is no significant change in the number and distribution of these clusters along the myofibres of a.l.d. and p.l.d. muscles. 9. These results are discussed with respect to motor end-plate formation in multiply and focally innervated embryo muscles, and in relation to the control of cholinergic proteins distribution and synthesis by muscle activity.